Screw and nut mechanism

ABSTRACT

In a screw and nut mechanism including transmission rollers the necessary displacement of the rollers one step backwards for each revolution of the nut with respect to the screw is brought about by at least one cam member, which operates between twin walls at the adjacent edge of cage, where the cam has a protected position and furthermore contacts the rollers about at their longitudinal axes.

I Umted States Patent 1 [111 3,744,332 Nilsson July 10, 1973 SCREW ANDNUT MECHANISM 2,831,363 4/1958 Lohr 74/4248 0 3,173,304 3/l965Strandgren 74/4248 C X [75] Walt" Pamlle, 3,214,991 ll/l965 Perrin74/424.8 c

, Sweden [73] Assignee: SKF Industrial Trading and PrimaryExaminer-Leonard H, Geri Development p y Attorney- Dexter N. Shaw,Eugene E. Renz et al. Amsterdam, Netherlands [22] Filed: Oct. 14, 197157 ABSTRACT PP 2 189, 01 In a screw and nut mechanism includingtransmission rollers the necessary displacement of the rollers one 52us. 01 74/424.8 c step backwards for each revhhih 0f the with [51] Int.Cl. F16]: 1/18 spect to the Screw is brought about by at least one cam58 Field of Search 74/424.8 c member, which P between twin walls thejacent edge of cage, where the cam has a protected po- [56] ReferencesCited sition and furthermore contacts the rollers about at UNITED STATESPATENTS their longitudinal axes.

3,595,094 7/1971 Lemor 74/424.8 C X 1 Claim, 4 Drawing Figures PatentedJuly 10, 1973 3,144,332

FIG. 2

Patented July 10, 1973 FIG.

1 SCREW AND NUT MECHANISM BACKGROUND OF THE INVENTION In order to reducethe friction between a nut and a screw it has been proposed to arrange anumber of rollers mounted in an annular cage between the nut and thescrew. The diameter of the nut is bigger an that of the screw, and therollers are provided with a number of peripherally closed ribs meshingwith the threads at the screw and at the nut, respectively.

During one revolution of the nut in relation to the screw the roller isdisplaced a distance in the axial direction corresponding to the pitchof the thread at the screw, and it is therefore necessary for eachrevolution to lift the roller out of engagement with the screw and thento push it back a distance corresponding to the pitch of the thread.This is brought about by means of cam members mounted on the nut. Theaim of the present invention is to propose a convenient embodiment ofthe cage and the cam member, which with reasonable manufacturing costsguarantees a safe governing of the rollers and simultaneously provides awell protected position for the cam elements.

SUMMARY OF THE INVENTION According to the invention the cage is providedwith twin walls, at least at one end portion, and a cam member at thenut is designed to operate between said twin walls in order to governthe movements of the rollers, said cam member during each revolution, inturn, contacting each roller about at the longitudinal axis thereof.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is an axial section through partofa screw and the nut thereon,

FIG. 2 is a perspective view of the cage and one cam element,

FIG. 3 is a section along line III III in FIG. 1, and

FIG. 4 shwos a development of a part of a cam member and the ends of therollers cooperating therewith.

DESCRIPTION OF THE PREFERRED EMBODIMENT The screw is provided withthreads at 11. The cooperating nut 12 is also provided with a screwthread 13, the diameter of which is that much bigger than that of thethread 11 that an annular space will be formed between the screw and thenut. A number of rollers 14 operate within this space, and is eachprovided with a number of ribs 15, which on the present occasion havesubstantially the same cross sectional shape as that of the threads atthe screw and at the nut.

These rollers serve as transmission elements between the nut and thescrew and will reduce the friction to a considerable degree. The rollers14 are fitted into an annual cage 16 provided with longitudinallyextending slots 17, into each of which a roller 14 is mounted withsufficient clearance to make possible an axial displacementcorresponding at least to the pitch of the thread at the screw and alsoradially a distance corresponding to the depth of the threads therein.

When the nut is turned in relation to the screw the rollers will bedisplaced along the screw and it is necessary for each revolution tobring the rollers back a distance corresponding to the pitch of thethread at the screw. A longitudinally extending groove 18 is formed inthe internal face of the nut 12. On each occasion when a roller passesthis groove it will be lifted out of engagement with the screw, and bepushed back by means of annular cam members 19 of the type best shown inFIG. 2. The cage 16 is provided with twin walls at its end portions, andone cam member is fitted at each end, between these walls. The cam path20 proper is located between these twin walls and will in this mannerobtain a protected position, and simultaneously be brought into contactwith the rollers about at the longitudinal axes thereof.

The annular portion 21 of the cam member located outside the cage ismore sturdy than the cam proper, and is provided with an externallythreaded part by means of which the cam member is retained in the nutand thus also will maintain the rollers in their proper positions.

A developement of a part of the cam path is shown in FIG. 4. It isevident that the profiles of the two cams at opposite ends of the cageshould be reversed in relation to each other, in such a manner that araising curve at one member will correspond to a retracting curve at theother member.

During its movenent around the screw a roller is mainly guided by meansof the threads in the screw and in the nut, respectively, and the actionof the cam elements will appear on each occasion when the roller isbrought to a position considing with that of the groove 18 in the nut.It is evident from FIGS. 2 and 4 that each cam member comprises asteeply sloping portion 22, and to each side thereof a straight portion23 and 24, respectively, upon which the rollers are prepared for thebringing out of, and into the thread 1 l at the screw.

Between the straight portion 24 and the strait portion 23 the cam pathruns around the member along a smoothly sinking curve 25, which does nothave any contact with the rollers.

The distance between the straight portions 23 and 24 and the inwardlyface of the annular portion 21 is denoted by a and b, respectively, andthe difference between these measures is equal to the pitch of thethread.

The cage may be manufacturded of an innerand an outer sleeve-shapedelement, in which the portions defining the longitudinal sides of theslots 17 are folded into engagement with each other to lock the sleeveelements in the desired relative position. The cage will on thisoccasion be provided with twin walls all along its extension, whichmeans a saving in weight compared to a solid cage, where the endportions only have been machined to provide the twin walls.

I claim:

1. In a screw and nut mechanism of the type where a cage supporting aplurality of rollers is fitted between an externally screw threadedshaft and an internally screw threaded nut enclosing the same, saidrollers being provided with peripheral ribs meshing with the.

thread of said shaft and nut respectively, to form transmission elementstherebetween, said rollers being operable during one revolution to bedisplaced in the axial direction of said shaft a distance correspondingto the pitch of its thread, and thereupon to be lifted free of itsengagement with said shaft and be brought into a longitudinal groove inthe nut, in which it is displaced backwards a distance corresponding tothe pitch of the thread of said shaft, and then to engage the latteronce more, the improvement comprising twin parallel walls at oppositeaxial ends of said cage, an annular cam member fitted at each end ofsaid cage and having a sleeve portion at one portion shaped as a camproper, extending inwardly between said twin walls to obtain being thecam proper into a position mating with that of the groove in the nut,said cams proper being reversely formed, one having a raising curve andthe contact with the juxtaposed ends of the rollers, and an 5 other aretracting curve-

1. In a screw and nut mechanism of the type where a cage supporting aplurality of rollers is fitted between an externally screw threadedshaft and an internally screw threaded nut enclosing the same, saidrollers being provided with peripheral ribs meshing with the thread ofsaid shaft and nut respectively, to form transmission elementstherebetween, said rollers being operable during one revolution to bedisplaced in the axial direction of said shaft a distance correspondingto the pitch of its thread, and thereupon to be lifted free of itsengagement with said shaft and be brought into a longitudinal groove inthe nut, in which it is displaced backwards a distance corresponding tothe pitch of the thread of said shaft, and then to engage the latteronce more, the improvement comprising twin parallel walls at oppositeaxial ends of said cage, an annular cam member fitted at each end ofsaid cage and having a sleeve portion at one portion shaped as a camproper, extending inwardly between said twin walls to obtain contactwith the juxtaposed ends of the rollers, and an external part forattachment to the nut in a position to being the cam proper into aposition mating with that of the groove in the nut, said cams properbeing reversely formed, one having a raising curve and the other aretracting curve.